The use of intraluminal prosthetic devices has been demonstrated to present an alternative to conventional vascular surgery. Intraluminal prosthetic devices are commonly used in the repair of aneurysms, as liners for vessels, or to provide mechanical support and prevent the collapse of stenosed or occluded vessels.
Intraluminal endovascular prosthetics involve the percutaneous insertion of a generally tubular prosthetic device, such as a stent, into a vessel or other tubular structure within the vascular system. The stent is typically delivered to a specific location inside the vascular system in a low profile (pre-deployed) state by a catheter. Once delivered to the desired location, the stent is deployed by expanding the stent into the vessel wall. The expanded stent typically has a diameter that is several times larger than the diameter of the stent in its compressed state. The expansion of the stent may be performed by several methods known in the art, such as by a mechanical expansion device (balloon catheter expansion stent) or by self-expansion.
The ideal stent utilizes a minimum width and wall thickness of the stent members to minimize thrombosis at the stent site after implantation. The ideal stent also possess sufficient hoop strength to resist elastic recoil of the vessel. To fulfill these requirements, many current tubular stents use a multiplicity of circumferential sets of strut members connected by either straight longitudinal connecting connectors or undulating longitudinal connecting connectors.
The circumferential sets of strut members are typically formed from a series of diagonal sections connected to curved or arc sections forming a closed-ring, zig-zag structure. This structure opens up as the stent expands to form the element in the stent that provides structural support for the vessel wall. A single strut member can be thought of as a diagonal section connected to a curved section within one of the circumferential sets of strut members. In current stent designs, these sets of strut members are formed from a single piece of metal having a uniform wall thickness, generally uniform strut width, as well as struts with uniform axial lengths. Similarly, the curved loop members are formed having a generally uniform wall thickness and generally uniform width.
Although the geometry of the stent members may be uniform, the strain experienced by each member under load is not. The “stress” applied to the stent across any cross section is defined as the force per unit area. These dimensions are those of pressure, and are equivalent to energy per unit volume. The stress applied to the stent includes forces experienced by the stent during deployment, and comprises the reactive force per unit area applied against the stent by the vessel wall. The resulting “strain” (deformation) that the stent experiences is defined as the fractional extension perpendicular to the cross section under consideration.
During deployment and in operation, each stent member experiences varying load along its length. In particular, the radial arc members are high in experienced loading compared to the remainder of the structure. When the stent members are all of uniform cross-sectional area, the resultant stress, and thus strain, varies. Accordingly, when a stent has members with a generally uniform cross-section, some stent members will be over designed in regions of lesser induced strain, which invariably results in a stiffer stent. At a minimum, each stent member must be designed to resist failure by having the member size (width and thickness) be sufficient to accommodate the maximum stress and/or strain experienced. Although a stent having strut or arc members with a uniform cross-sectional area will function, when the width of the members are increased to add strength or radio-opacity, the sets of strut members will experience increased stress and/or strain upon expansion. High stress and/or strain can cause cracking of the metal and potential fatigue failure of the stent under the cyclic stress of a beating heart.
Cyclic fatigue failure is particularly important as the heart beats, and hence the arteries “pulse”, at typically 70 plus times per minute—some 40 million times per year—necessitating that these devices are designed to last in excess of 108 loading cycles for a 10-year life. Presently, designs are both physically tested and analytically evaluated to ensure acceptable stress and strain levels are achievable based on physiologic loading considerations. This is typically achieved using the traditional stress/strain-life (S-N) approach, where design and life prediction rely on a combination of numerical stress predictions as well as experimentally-determined relationships between the applied stress or strain and the total life of the component. Fatigue loading for the purpose of this description includes, but is not limited to, axial loading, bending, torsional/twisting loading of the stent, individually and/or in combination. One of skill in the art would understand that other fatigue loading conditions can also be considered using the fatigue methodology described as part of this invention.
Typically, finite-element analysis (FEA) methodologies have been utilized to compute the stresses and/or strains and to analyze fatigue safety of stents for vascular applications within the human body. This traditional stress/strain-life approach to fatigue analysis, however, only considers geometry changes that are uniform in nature in order to achieve an acceptable stress and/or strain state, and does not consider optimization of shape to achieve near uniform stress and/or strain along the structural member. By uniformity of stresses, a uniformity of “fatigue safety factor” is implied. Here fatigue safety factor refers to a numerical function calculated from the mean and alternating stresses measured during the simulated fatigue cycle. In addition, the presence of flaws in the structure or the effect of the propagation of such flaws on stent life are usually not considered. Moreover, optimization of the geometry considering flaws in the stent structure or the effect of the propagation of such flaws has not been implemented.
What is needed is a stent design where the structural members experience near uniform stress and/or strain along the member, thereby maximizing fatigue safety factor and/or minimizing peak strain, and analytical methods to define and optimize the design, both with or without imperfections. One such resulting design contemplates stent members with varying cross-sections and strut members having different axial lengths. The design produces near uniform stress and/or strain for a given loading condition with or without the presence of defects or imperfections. The design also allows for greater flexibility, conformability, and offers a smaller crimping profile.